The present invention concerns a hook control apparatus for a shed forming machine such as a Jacquard machine.
One form of hook control apparatus for a shed forming machine, as disclosed in German patent specification No. 3 713 832, comprises lift blades and hooks which can be selectively entrained by the lift blades. The hooks are adapted to pivot in a bottom shed position between a hooking engagement position in which they can be engaged by the blades and a non-hooking position in which they are not engageable by the blades. The hooks are supported in the bottom shed position on a hook bottom member and are biased towards the hooking engagement position. The hooks can be arrested and retained in their non-hooking position by suitable magnetic means such as electromagnetic devices. In that arrangement therefore the magnetic means are disposed in the region of the pivotal movement of the hooks at the level of the bottom shed position thereof, and the hooks are in the form of magnet armature members. However, a disadvantage with that design configuration is that the magnetic means are disposed at locations which do not afford ready access thereto. The electric leads required for electrical actuation of the magnetic means each have to be brought out of the region of the bottom shed position and the region of the lift blades respectively. The amount of space available for installation of the magnetic means in the arrangement is very small, unless the disadvantage of the machine being of very substantial depth can be tolerated. Further problems with that arrangement arise out of the consideration that the magnetic force for actuation of the hooks is required to be sufficiently high while on the other hand the magnetic leakage of the magnetic means needs to be at a low level.
In the above-indicated arrangement in which the hooks constitute magnet armature members, each hook consists entirely of iron for use as the armature member. As the lift blades generally comprise steel, it is necessary to provide lubrication at the respective point of contact between each hook and the co-operating lift blade, which can be a further disadvantage in such a machine. If however use is made of another possible way of providing the armature member, in which each hook for example comprises a plastic material and the armature member is in the form of an iron member which is incorporated into the plastic hook as by being included in the injection molding forming the latter, then that design configuration is expensive to manufacture. Further problems arise due to the differences in the coefficients of thermal expansion of the plastic and ferrous materials used, which can have a disadvantageous effect in regard to the flatness or straightness of the contact surfaces on the armature members, which are intended to come to bear against the respective electromagnetic means. It will be appreciated that, if the armature members do not lie in full flat surface contact against the magnetic means, the armature member retaining force is considerably reduced, with the result of a reduction in operating reliability of the system. Distortion caused by differences in thermal expansion of the plastic hook and the iron armature member may prevent full flat surface contact between the respective armature member and the co-operating magnetic device.